1. Field of the Invention
The present invention relates to an internal combustion engine installed in an engine compartment covered with an engine cover, and an outboard motor provided with the internal combustion engine. More specifically, the present invention relates to a construction of the intake air passage of an internal combustion engine.
2. Description of the Related Art
There are various known techniques for preventing water from flowing together with the combustion air into the combustion chambers of an internal combustion engine. Particularly, an internal combustion engine included in a marine propulsion apparatus, such as an outboard motor, is required to be compact in construction and excellent in water eliminating function. For instance, a technique applied to an internal combustion engine included in an outboard motor disclosed in Japan Patent No. 2726718 uses a U-shaped intake air passage to prevent water from flowing into the internal combustion engine. In a technique disclosed in JP 2002-138912 A, the inlet of an intake air passage opens into an engine compartment, and the outlet of an air guide passage for carrying air into the engine compartment opens downward in the engine compartment.
Hot air heated by heat generated by an internal combustion engine in an engine compartment tends to flow through the inlet of an intake air passage into the intake air passage when the inlet of the intake air passage opens in an engine compartment encasing the internal combustion engine. Consequently, charging efficiency at which the internal combustion engine is charged with air decreases. To improve the charging efficiency, a technique disclosed in, for example, JP 59-120598 A opens the inlet of an intake air passage near and at the outside of an engine compartment.
When air outlets are formed on the way of an up passage, in which reversed descending combustion air that has flowed through a down passage of a U-shaped intake air passage flows up in ascending currents, it is possible that water contained in the combustion air which could not have been removed by the U-shaped intake air passage flows together with the combustion air through some of the air outlets, such as the lowermost air outlet at the lowest position among the air outlets, into the combustion chamber. Thus, a sufficient water separating efficiency cannot be performed by the U-shaped intake air passage.
Water separating efficiency needs to be improved when the air inlet of an intake air passage is opened into a space near an outside air intake space to improve the charging efficiency because water can easily flow into the intake air passage in such a state, as compared with a state where the air inlet is opened into an engine compartment.
When the air inlet of an intake air passage is connected to an engine compartment, the pressure of air in the engine compartment is caused to vary by intake pulsation, and sometimes the variation of air pressure in the engine compartment causes an engine cover to vibrate and to generate noise.
The present invention has been made under such circumstances and it is therefore an object of the present invention to improve the effect on preventing water from flowing into the combustion chambers of an internal combustion engine. Another object of the present invention is to improve the charging efficiency and to prevent the vibration of an engine cover due to intake pulsation. A further object of the present invention is to prevent a throttle valve from being wet with water and to increase the passage area of a first down passage, while compactly arranging a throttle valve and an intake air passage. A still further object of the present invention is to improve the water trapping effect and charging efficiency of an outboard motor and to prevent the noise generation of an engine cover due to intake pulsation.